1. Field of the Invention
The present invention relates to an AGC voltage correction circuit preferable for a stage of radio-frequency amplification or intermediate-frequency amplification in a portable telephone of, for example, a code division multi-access (CDMA) type.
2. Description of the Related Art
In general, a CDMA type portable telephone has a variable gain amplification circuit (hereinafter, a variable amplification circuit) capable of varying a gain of 80 dB or more included in a reception unit or transmission unit for the purpose of retaining uniformly a signal strength for reception and the strength of a signal reaching a base station. FIG. 9 shows a typical variable amplification circuit formed with a differential amplification circuit composed of amplification transistors Q17 and 018, current source transistor Ql9, and load resistors R8 and R9 connected to transistors Q17 and Q18 respectively. Also shown are input terminal IN, output terminal OUT, AGC (gain control) voltage V.sub.AGC, and supply voltage V.sub.cc.
Gain PC dB! in the foregoing circuitry has the following relationship: EQU PG.oe butted.20 log (I1* q/kT) (1)
The following relationship is also established: EQU I1 .oe butted.Is*exp{V.sub.AGC *q/(kT)} (2)
where q denotes a unit charge of an electron, k denotes a Boltzmann's constant, T denotes an absolute temperature, and Is denotes an inverse saturation current.
The expression (1) is concerned with transistors Q17 and Q18, and stipulates that when a current (collector current of the transistor Q19) supplied from a constant current source is controlled exponentially, power gain PG dB! varies linearly. FIG. 10 shows experimental values indicating the relationship between current I1 and gain PG dB! in the circuit shown in FIG. 9 relative to different frequencies. According to the expression (1), when collector current I1 is changed to be ten times larger, gain PG varies by 20 dB. The results of an experiment also demonstrate like the expression (1 ) that when current I1 changes from 0.1 mA to 1 mA, gain PG varies by approximately 20 dB. Transistor Q19 employed herein causes a current of 20 mA to flow at cutoff frequency Ft of a maximum value. The expression (1) is established with a current that is a one-tenth of the current flowing at cutoff frequency Ft of the maximum value.
The expression (2) stipulates that collector current I1 changes exponentially relative to AGC voltage V.sub.AGC that varies linearly. Power gain PG dB! of the variable amplification circuit shown in FIG. 9 also varies linearly relative to AGC voltage V.sub.AGC, which varies linearly, with a current that is a one-tenth or smaller of the current flowing at cutoff frequency Ft of the maximum value, that is, 2 mA or less.
Next, a variation dependent on temperature will be described. According to the expression (1), when temperature changes from 25.degree. C. (T=298) to 75.degree. C. (T=348), even if collector current I1 of the current source transistor Q19 is held constant, a gain decreases by approximately 1.4 dB (cause 1). Moreover, the gain rises with a temperature change in a negative direction. For realizing a gain or 80 dB or more in a variable amplification circuit employed in an ordinary CDMA type portable telephone, a circuit made by cascading three or four stages of variable amplification circuits each like the one shown in FIG. 9 is usually adopted. For a temperature change from 25.degree. C. to 75.degree. C., the gain varies by 4 to 5 dB (number of stages * 1.4 dB per 50.degree. C.).
In consideration of the fact that inverse saturation current Is has a temperature characteristic, simulation is carried out using SPICE, and collector current I1 of a standard bipolar transistor relative to AGC voltage V.sub.AGC is calculated using the expression (2). The resultant values are as shown in FIG. 11. As seen from FIG. 11, collector current II varies greatly with a temperature change. For example, even when AGC voltage V.sub.AGC remains constant, if a temperature changes from 25.degree. C. to 75.degree. C., collector current I1 changes to be ten times or more larger. As a result, the gains of transistors Q17 and Q18 vary by 20 dB or more (cause 2). This phenomenon leads to a variation by 60 dB (3*20 dB per 50.degree. C.) of the gain of the triple-stage variable amplification circuit.
Moreover, a rate of change in collector current I1 relative to AGC voltage V.sub.AGC, or in other words, a gain variation rate (gain slope) varies depending on temperature (cause 3).
When bipolar transistors are used to construct a variable amplification circuit capable of varying a gain thereof within a range of 80 dB or more which is necessary for a CDMA type portable telephone, there arise problems set forth below.
1. Gain PG of the variable amplification circuit varies greatly depending on temperature because of the temperature dependency of the gains of amplification transistors Q17 and Q18 (cause 1) and the temperature dependency of the current source transistor Q19 (cause 2).
2. The gain variation rate (gain slope) relative to AGC voltage V.sub.AGC varies greatly depending on temperature because of the temperature dependency of the current source transistor Q19 (cause 3).
3. The gain slope has, as shown in FIG. 10, scales in an area, which are associated with larger values of collector current I1, spaced narrowly, and the linearity of the gain slope deteriorates in the area. Moreover, in an area of scales associated with smaller values of collector current I1, the gain decreases sharply because of the frequency characteristic of cutoff frequency Ft (that is, cutoff frequency Ft is proportional to collector current I1), and the linearity of the gain slope deteriorates.
4. When three or four stages of variable amplification circuits are included to realize a gain of 80 dB or more, gains of stages become mutually different because of differences among transistors or resistors. For correcting the difference, if a variable resistor or any other adjusting means is included in an input stage or output stage, isolation of radio-frequency circuit elements becomes inefficient. Eventually, a gain variable range gets narrower or the linearity of a gain slope deteriorates.